Highly lordosed fusion cage

ABSTRACT

A fusion cage has a first component that defines an outside surface that is configured to engage a vertebral endplate, and an interior surface. The fusion cage has a second component that defines first and second opposed surfaces. One of the first and second opposed surfaces can mate with the interior surface of the first component. The fusion cage can include vertical and lateral throughholes adapted to enhance fusion.

CONTINUING DATA

This divisional application claims priority from co-pending U.S. Ser.No. 11/768,636, entitled “Highly Lordosed Fusion Cage”, filed Jun. 26,2007, by Hawkins et al., the specification of which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The natural intervertebral disc contains a jelly-like nucleus pulposussurrounded by a fibrous annulus fibrosus. Under an axial load, thenucleus pulposus compresses and radially transfers that load to theannulus fibrosus. The laminated nature of the annulus fibrosus providesit with a high tensile strength and so allows it to expand radially inresponse to this transferred load.

In a healthy intervertebral disc, cells within the nucleus pulposusproduce an extracellular matrix (ECM) containing a high percentage ofproteoglycans. These proteoglycans contain sulfated functional groupsthat retain water, thereby providing the nucleus pulposus within itscushioning qualities. These nucleus pulposus cells may also secretesmall amounts of cytokines such as interleukin-1β and TNF-α as well asmatrix metalloproteinases (“MMPs”). These cytokines and MMPs helpregulate the metabolism of the nucleus pulposus cells.

In some instances of disc degeneration disease (DDD), gradualdegeneration of the intervetebral disc is caused by mechanicalinstabilities in other portions of the spine. In these instances,increased loads and pressures on the nucleus pulposus cause the cellswithin the disc (or invading macrophases) to emit larger than normalamounts of the above-mentioned cytokines. In other instances of DDD,genetic factors or apoptosis can also cause the cells within the nucleuspulposus to emit toxic amounts of these cytokines and MMPs. In someinstances, the pumping action of the disc may malfunction (due to, forexample, a decrease in the proteoglycan concentration within the nucleuspulposus), thereby retarding the flow of nutrients into the disc as wellas the flow of waste products out of the disc. This reduced capacity toeliminate waste may result in the accumulation of high levels of toxinsthat may cause nerve irritation and pain.

As DDD progresses, toxic levels of the cytokines and MMPs present in thenucleus pulposus begin to degrade the extracellular matrix, inparticular, the MMPs (as mediated by the cytokines) begin cleaving thewater-retaining portions of the proteoglycans, thereby reducing itswater-retaining capabilities. This degradation leads to a less flexiblenucleus pulposus, and so changes the loading pattern within the disc,thereby possibly causing delamination of the annulus fibrosus. Thesechanges cause more mechanical instability, thereby causing the cells toemit even more cytokines, thereby upregulating MMPs. As this destructivecascade continues and DDD further progresses, the disc begins to bulge(“a herniated disc”), and then ultimately ruptures, causing the nucleuspulposus to contact the spinal cord and produce pain.

One proposed method of managing these problems is to remove theproblematic disc and replace it with a porous device that restores discheight and allows for bone growth therethrough for the fusion of theadjacent vertebrae. These devices are commonly called “fusion devices”.

U.S. Pat. No. 5,865,848 (“Baker”) discloses a two piece intervertebralfusion cage having a ramp. Baker describes a intervertebral spacercomprised of two components. The two portions have opposed flangesconnected by a screw to effect translation, and complimentary slopes.The components are inserted together in a collapsed condition.Post-insertion translation of one component relative to the othercreates an expanded condition and the desired distraction.

US Published Patent Application 2004/0230309 (“DePuy Spine”) discloses atwo piece intervertebral fusion cage having a ramp. See FIG. 14D.

US Published Patent Application Nos. US2003/0135275 and 2003/0139812(collectively, “Garcia”) disclose a two-piece implant formed by upperand lower halves, wherein the inner surfaces of the two halves form adovetail joint that runs along a transverse axis of the implant.

SUMMARY OF THE INVENTION

The present invention is directed to a two-piece intervertebral fusioncage, comprising:

-   -   a) an upper component having a first outside surface adapted for        gripping an upper vertebral endplate and a first interior        surface,    -   b) a lower component having a second outside surface adapted for        gripping a lower vertebral endplate and a second interior        surface,        wherein the interior surfaces mate.

One advantage of such a cage is its easy insertion. In a first step, thelower component is inserted into the disc space and is held in place.The first step confirms placement of the implant and its footprint. In asecond step, the upper component is inserted into the disc space bysliding its interior surface along the opposed interior surface of thelower component. This two-step method of insertion eliminates the needto provide an independent distraction means, such as the use of animpaction hammer, and an independent trialing means. It also providesmechanical leverage in the cage to facilitate the creation of lordosis.

A second advantage of such a cage is its impact on patient safety. Thegradual nature of the distraction of the disc space provided by thistwo-step insertion procedure should also reduce the possibility ofover-distraction, which can cause neural damage. It also eliminateshammer-induced sudden impaction during cage insertion, thereby reducingcage failures, over-insertion and anterior damage. Lastly, the smallerheight of the annular defect produced during device insertion aids inpreventing device expulsion.

In a first aspect of the present invention, the outside surface of atleast one of the components is substantially parallel to itscorresponding interior surface. This provides the advantage of ease ofinsertion through a small annular defect/incision. Simply, one componentcan determine height and the other component can determine lordosis.

In a second aspect of the present invention, each component has athroughhole extending from its outside surface to its interior surface,and the interior surfaces of these components mate to align the firstand second throughholes. This alignment provides a path for bone growththrough the vertical dimension of the device, thereby facilitatingfusion between the vertebral endplates.

In a third aspect of the present invention, each component has opposedsidewalls and at least one of the components has a lateral throughholeextending from a sidewall thereof to its opposed sidewall. This lateralthroughhole provides a path for increased vascularization, increaseddiffusion, and bone growth through the lateral dimension of the device,thereby facilitating fusion between the vertebral endplates.

In a fourth aspect of the present invention, each component has adovetail feature extending along the longitudinal axis of its interiorsurface and the dovetail features of the corresponding components matealong the longitudinal axes. This mating of dovetails provides for alocking of the mated upper and lower components and increases theassurance that the mated components will not disengage in vivo.

In a fifth aspect of the present invention, the upper component has atapered distal end, preferably a bulleted nose. This tapered distal endallows for easy distraction of the opposed vertebral endplates uponinsertion of the upper component into the disc space.

DESCRIPTION OF THE FIGURES

FIGS. 1a-1d disclose a cage of the present invention having matingdovetail features and aligned vertical throughholes.

FIGS. 1e-1g disclose the sequential insertion and assembly of thecomponents of the device into the disc space.

FIG. 2 discloses a cage of the present invention having a lateralthroughhole extending from a sidewall thereof to its opposed sidewall,and an outside surfaces that is substantially parallel to itscorresponding interior surface.

FIG. 3 discloses an exploded cage of the present invention having abulleted distal nose, along with an assembled cage attached to aninsertion instrument.

FIG. 4 discloses an interior surface of the lower component that isangled with respect to its corresponding outer surface, thus creatinglordosis.

FIG. 5a discloses a cage of the present invention wherein an insidesurface of the first component has a recess, and an inside surface ofthe second component has an extension, wherein the recess and extensionmate to provide nesting of the components.

FIGS. 5b and 5c disclose a method of inserting the device of FIG. 5 a.

FIG. 6 discloses a cage of the present invention wherein one of thecomponents has extending side walls that create a housing adapted tocapture the other component.

FIGS. 7a and 7b disclose the device of the present invention attached toan insertion instrument.

FIGS. 8a-8c disclose arcuate cages of the present invention.

FIG. 8d discloses an arcuate cage inserted into a disc space.

DETAILED DESCRIPTION OF THE FIGURES

For the purposes of the present invention, the terms “inner surface”,“inside surface” “interior surface are interchangeable, as are the terms“exterior surface”, “outer surface” and “outside surface”.

In general, the cage possesses a two-piece structure with hard-endplatesand a locking means that is compatible with MIS techniques. Both the topand bottom portions have an interior and exterior surface (FIG. 1),where the exterior surface interfaces with the vertebra.

Now referring to FIGS. 1a-1d , there is provided an intervertebralfusion cage, comprising:

-   -   a) an upper component 1 having a first outside surface 3 adapted        for gripping an upper vertebral endplate, a first interior        surface 5 having a first longitudinal axis, a first throughhole        7 extending from the outside surface to the first interior        surface, and a first dovetail feature (not shown) extending        along the first longitudinal axis, and    -   b) a lower component 11 having a second outside surface 13        adapted for gripping a lower vertebral endplate, a second        interior surface 15 having a second longitudinal axis, a second        throughhole 17 extending from the outside surface to the second        interior surface, and a second dovetail feature 19 extending        along the first longitudinal axis,        wherein the interior surfaces mate to align the first and second        throughholes.

The device of FIG. 1 possesses mating dovetail features on its interiorsurfaces. These features help maintain the device in its assembled form.The device of FIG. 1 also possesses aligned vertical throughholesthrough each component in its assembled form. These aligned holes helpprovide desirable bone growth through the device.

In use, the component halves of the device of the present invention areinserted into the disc space is a sequential fashion and are assembledin situ. Now referring to FIG. 1e , the lower component 11 is firstinserted into the disc space, with the tapered portion pointingposteriorly, so that it rests upon the floor of the disc space. Thereduced height of the lower component allows it to be inserted withoutany need for distraction. Now referring to FIG. 1f , the inside surfaceof the upper component is mated to the inside surface of the lowercomponent and advanced into the disc space. Because the height of thecombined components is greater than the disc space, and because the noseof the upper component is tapered, gradual insertion of the uppercomponent into the disc space in this manner provides a gradualdistraction of the disc space. Now referring to FIG. 1g , the assembledcomponent is located within the disc space.

In one embodiment, the interior surface of the top and/or bottom portionis generally parallel to their exterior surface. Now referring to FIG.2, there is provided an intervertebral fusion cage, comprising:

-   -   a) an upper component 21 having a first outside surface 23        adapted for gripping an upper vertebral endplate and a first        interior surface 25, a first sidewall 27 and a second opposed        sidewall (not shown),    -   b) a lower component 31 having a second outside surface 33        adapted for gripping a lower vertebral endplate and a second        interior surface 35, a third sidewall 37 and a second fourth        sidewall (not shown),        wherein the interior surfaces mate,        wherein at least one of the components has a lateral throughhole        39 extending from a sidewall thereof to its opposed sidewall,        and        wherein at least one of the outside surfaces is substantially        parallel to its corresponding interior surface.

An alternative embodiment would have sloped sides and/or non-parallelthroughhole walls. These walls may be curved inwards (concave) or bowedoutward (convex). Such an embodiment would increase the mechanicalstructural stability of the assembled device. Additionally, increasingthe devices' mating surface areas would give additional room for thedovetail and locking features. Simultaneously, increased mating surfaceareas would decrease normal and shear load distributions at the matingsurfaces, thereby decreasing likelihood of mechanical failures andfurther minimizing potential for the generation of particulate weardebris.

The device of FIG. 2 possesses a component that has parallel outside andinterior surface. The device of FIG. 2 also possesses aligned lateralthroughholes through at least one of its components. These holes help indesirable bone growth through the device.

Now referring to FIG. 3, there is provided the upper 41 and lower 51halves of the intervertebral fusion cage of the present invention, alongwith an assembled cage 42 attached to an insertion instrument 44. Theintervertebral fusion cage comprises:

-   -   a) an upper component 41 having a first outside surface 43        adapted for gripping an upper vertebral endplate, a first        interior surface 45 having a recess 46, and a tapered distal end        47,    -   b) a lower component 51 having a second outside surface 53        adapted for gripping a lower vertebral endplate and a second        interior surface 55 having a projection 56 and a taper distal        end 58,        wherein the interior surfaces mate so that the recess and        projection form a stop 57 and the tapered distal ends form a        bullet nose 60.

The upper component of the device of FIG. 3 possesses a bullet distalnose. This bullet nose helps the upper component distract the disc spaceas it is inserted into the disc space. The insertion instrument 44includes a rail 48, a top pusher 50, and a bottom rod 52 having aprojection 54 that mates with a recess 38 in the lower component of thecage.

In another embodiment, the interior surface 63 of the lower component 61creates a ramp having an angle α with respect to its corresponding outersurface 65 (FIG. 4). The upper component 67 of the implant translatesalong this ramp, creating lordosis.

Because the two pieces of the cage are inserted sequentially into thedisc space, the “insertion” height of the assembly and thus the heightof the annular defect required during the insertion is approximately onehalf of the assembled device height.

Now referring to FIG. 5a , there is provided an intervertebral fusioncage, comprising:

-   -   a) a first component 71 having a first outside surface 73        adapted for gripping a first vertebral endplate, a first        interior surface 75, and a recess 77 in the first interior        surface extending toward the first outside surface,    -   b) a second component 81 having a second outside surface 83        adapted for gripping a second vertebral endplate, a second        interior surface 85, and an extension 87 extending from the        second interior surface toward the first interior surface,        wherein the recess and extension of the interior surfaces mate        to provide nesting of the components.

Now referring to FIGS. 5b and 5c , there is provided a method ofinserting the nested cage of FIG. 5a : Place the baseplate 81 in theinterbody disc space. Position and slide the overrider plate 71 onto andover the baseplate 81. Initially, the overrider nose 78 will slideacross the base plate's interior surface 87 thereby creating a slightover-distraction of the disc space. As the overrider's posterior aspect75 approaches the baseplate's posterior niche 85, the anterior overridernose 78 will fall into the anterior niche 88 thereby providing apositive stop and locking mechanism to seat the overrider plate 71 ontothe baseplate 81.

The preferred method for the order of insertion of the two-piece cage ofthe present invention is now disclosed. The first step can be placementof the first component of the device against the inferior vertebralbody. This is often followed by insertion and eventual assembly of thesecond component onto and over the first component. The first componentshould contain a lordosed or angled component such that the surface overwhich the second component is inserted is substantially parallel to thesuperior vertebral endplate.

Alternatively, the method of inserting the first component withsubstantially parallel bone contacting and interior surfaces requiresthat the second component to contain a lordotic angle. As this two-pieceassembled cage is typically inserted from a posterior approach, and theangle of the interbody cavity widens anteriorly, insertion of the secondcomponent with a lordotic angle requires over-distraction of theposterior aspect of the interbody space to accommodate the larger/talleranterior aspect of the second component. This over-distraction and theassociated increased insertion force is not associated with thepreferred method where the lordosed component is inserted first followedby the height-restoring component with substantially parallel sides.

Now referring to FIG. 6, there is provided an intervertebral fusioncage, comprising:

-   -   a) a first component 101 having a first outside surface 103        adapted for gripping an upper vertebral endplate and a first        interior surface 105, a first sidewall 107 extending between the        first outside surface and the first interior surface and having        an outer surface (not shown) and a second opposed sidewall 109        extending between the first outside surface and the first        interior surface and having an outer surface (not shown),    -   b) a housing component 111 having a second outside surface 113        adapted for gripping a lower vertebral endplate and a second        interior surface 115, a third sidewall 117 extending from the        second interior surface and away from the second outside surface        and having an inner surface 118, and a fourth sidewall 119        extending from the second interior surface and away from the        second outside surface and having an inner surface 120,        wherein the interior surfaces mate, and        wherein the outer surfaces of the first component mates with        their respective inner surfaces of the housing component.

In FIG. 6, one of the components has extending side walls that create ahousing adapted to capture the other component. The advantage of thisembodiment is that it possesses an enhanced dovetail creating enhancedstability.

Now referring to FIGS. 7a-b , there is provided a device for insertingthe components of the present invention into the disc space. FIG. 7adiscloses the disposition of the instrument during insertion of theinferior component into the disc space. FIG. 7b discloses thedisposition of the device when the upper component is advanced into thedisc space to the point where it rides on top of the lower component andforms the assembly.

The endplates may have teeth to prevent migration and perforations toallow bone growth.

Radiographic markers can be incorporated to allow intra- orpost-operative visualization.

Additionally, the outside surfaces of the superior and inferior portionscan be designed with varying geometries to allow for their customizationto the patient's anatomy, thereby minimizing the risk of subsidence. Forinstance, the outside surface of the superior half may have a convexdome while the outside surfaces of the inferior half may be flat,convex, concave or serpentine (concave posterior and convex anterior).

In an alternative embodiment, an arcuate sliding mating pathway iscontemplated. FIGS. 8a-8c disclose arcuate cages of the presentinvention forming a lordotic angle 201 (FIG. 8a ), no angle 203 (FIG. 8b) and a kyphotic angle 205 (FIG. 8c ). FIG. 8d discloses a lordoticarcuate cage 201 inserted into a disc space.

The benefit to assembling the two cage halves using an arcuate assemblypathway is the potential to provide in situ determination of lordoticangle. Also, the arcuate pathway would embody a mechanism allowing acontinuously variable lordotic angle (as opposed to discrete lordoticangles represented by assembly of different superior or inferior cagecomponents). The arcuate mating mechanism requires that one half of thecage contains a convex mating surface while the other half contains aconcave mating surface with an arcuate geometry that exactly mimics theinverse of the opposite curve. The assembled halves would represent afixed radial distance about some center of rotation (the geometriccentroid of the arcuate pathway). This fixed radial distance representsthe cage height. As the superior cage half slides along the arcuatemating surface of the inferior cage half, the lordotic angle of theassembly will vary continuously. Detents, stops, or teeth can beinserted into the arcuate pathway to create discrete increments oflordotic angles along the continuous arcuate pathway, if this is apreferred further embodiment.

The arcuate pathway enables similar seating features compared to theplanar device mating pathway—dovetails, keyways, detents, snap-fits, setscrews, etc.

The two halves of the device may be secured together by various means.For instance, a dovetail feature may be incorporated into the interiorsurfaces of the top and bottom portions for ease of insertion, as shownin FIGS. 1a-f , 2 and 3. The two components may also be stacked andnested together without a dovetail, as shown in FIGS. 5a-c , or thesuperior/inferior half may have side walls creating a housing thatcaptures the other portion, as shown in FIG. 6. The two portions of thedevice may be locked together in a variety of locking means includingbut not limited to Morse taper locks, positive stop(s), ultrasonicwelding, snap mechanism(s), a set screw, a clip, a collet, cams, etc.Additional design features can be included to aid in seating the twohalves: mechanical keys, a threaded nut can screw onto threaded featureson the posterior aspect of each half, and a mechanical channel can beincorporated into the design for the use of curing compounds likeadhesives and grouts.

Both components of the device may also incorporate a variety of holdingmeans to assist during the insertion of the device. These holding meansmay be located on the interior or exterior surfaces as well as along thesidewalls. For example, the top and/or bottom portion may have threadedholes, divots, or slots to provide for secure holding and cage supportduring insertion, placement and assembly.

After placing the inferior portion, the superior portion can be insertedby several means to expand the overall device height and provideappropriate lordosis or kyphosis.

The superior and inferior portions are generally hollow to provide forfilling with various osteogenic fillers and can be porous to allow forgraft filling, bony ingrowth and spinal fusion. Lateral openings canalso be incorporated to increase vascularization of the osteogenicfillers as well as to provide post-operative visualization of the bonyfusion process. Filling can be done preoperatively or intraoperatively,as a through hole into the wedge can facilitate filling of the entireconstruct in situ

Unlike traditional single-piece cages, the two-piece assembled cagerequires sliding articulation of two half-cages packed with bone. Bonepacked within a cage is typically held in place using friction forces.The sliding assembly mechanisms described could potentially dislodgepacked bone graft during cage insertion and/or assembly. To mitigate thedislodgement of bone chips or the potential for sliding-interference ofthe bone chips, bonding a resorbable lamina of material between the twohalfs is proposed. Such a lamina could be placed on the interior surface45 of the upper half 43 (see FIG. 3). Such a resorbable member could beapplied to all cage openings but is of particular utility to preventdislodgement or interference of the bone graft during sliding assemblyof the two halves.

The present invention also offers novel trialing methods. The inferioror superior portion of the implant device can be inserted alone toconfirm disc space clearance and device placement, and a trial of thesuperior component can be placed upon the inserted component to confirmdisc height, lordosis, and placement.

Because these cages reduce the profile required for their insertion,they allow for implantation through a cannula that may be smaller thanthe conventional cannula.

The endplates can be made of any structural biocompatible materialincluding resorbable (PLA, PLGA, etc.), non-resorbable polymers (CFRP,PEEK, UHMWPE, PDS), metallics (SS, Ti-6Al-4V, CoCr, etc.), as well asmaterials that are designed to encourage bony regeneration (allograft,bone substitute-loaded polymers, growth factor-loaded polymers,ceramics, etc.). The materials for the upper and lower components arebiocompatible and generally similar to those disclosed in the prior art.Examples of such materials are metal, PEEK and ceramic.

In preferred embodiments, each of the upper and lower components ismanufactured from a material that possesses the desirable strength andstiffness characteristics for use as a fusion cage component.

These components of the present invention may be made from anynon-resorbable material appropriate for human surgical implantation,including but not limited to, surgically appropriate metals, andnon-metallic materials, such as carbon fiber composites, polymers andceramics.

In some embodiments, the cage material is selected from the groupconsisting of PEEK, ceramic and metallic. The cage material ispreferably selected from the group consisting of metal and composite(such as PEEK/carbon fiber).

If a metal is chosen as the material of construction for a component,then the metal is preferably selected from the group consisting oftitanium, titanium alloys (such as Ti-6Al-4V), chrome alloys (such asCrCo or Cr—Co—Mo) and stainless steel.

If a polymer is chosen as a material of construction for a component,then the polymer is preferably selected from the group consisting ofpolyesters, (particularly aromatic esters such as polyalkyleneterephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE;polyarylethyl ketone PAEK; polyphenylene and mixtures thereof.

If a ceramic is chosen as the material of construction for a component,then the ceramic is preferably selected from the group consisting ofalumina, zirconia and mixtures thereof. It is preferred to select analumina-zirconia ceramic, such as BIOLOX Delta™, available from CeramTecof Plochingen, Germany. Depending on the material chosen, a smoothsurface coating may be provided thereon to improve performance andreduce particulate wear debris.

In some embodiments, the cage member comprises PEEK. In others, it is aceramic.

In some embodiments, the first component consists essentially of ametallic material, preferably a titanium alloy or a chrome-cobalt alloy.In some embodiments, the second component consists essentially of thesame metallic material as the first plate.

In some embodiments, the components are made of a stainless steel alloy,preferably BioDur® CCM Plus® Alloy available from Carpenter SpecialtyAlloys, Carpenter Technology Corporation of Wyomissing, Pa. In someembodiments, the outer surfaces of the components are coated with asintered beadcoating, preferably Porocoat™, available from DePuyOrthopaedics of Warsaw, Ind.

In some embodiments, the components are made from a composite comprisingcarbon fiber. Composites comprising carbon fiber are advantageous inthat they typically have a strength and stiffness that is superior toneat polymer materials such as a polyarylethyl ketone PAEK. In someembodiments, each component is made from a polymer composite such as aPEKK-carbon fiber composite.

Preferably, the composite comprising carbon fiber further comprises apolymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). Morepreferably, the PAEK is selected from the group consisting ofpolyetherether ketone (PEEK), polyether ketone ketone (PEKK) andpolyether ketone (PEK). In preferred embodiments, the PAEK is PEEK.

In some embodiments, the carbon fiber comprises between 1 vol % and 60vol % (more preferably, between 10 vol % and 50 vol %) of the composite.In some embodiments, the polymer and carbon fibers are homogeneouslymixed. In others, the material is a laminate. In some embodiments, thecarbon fiber is present in a chopped state. Preferably, the choppedcarbon fibers have a median length of between 1 mm and 12 mm, morepreferably between 4.5 mm and 7.5 mm. In some embodiments, the carbonfiber is present as continuous strands.

In especially preferred embodiments, the composite comprises:

-   -   a) 40-99% (more preferably, 60-80 vol %) polyarylethyl ketone        (PAEK), and    -   b) 1-60% (more preferably, 20-40 vol %) carbon fiber,        wherein the polyarylethyl ketone (PAEK) is selected from the        group consisting of polyetherether ketone (PEEK), polyether        ketone ketone (PEKK) and polyether ketone (PEK).

In some embodiments, the composite consists essentially of PAEK andcarbon fiber. More preferably, the composite comprises 60-80 wt % PAEKand 20-40 wt % carbon fiber. Still more preferably the compositecomprises 65-75 wt % PAEK and 25-35 wt % carbon fiber.

Although the present invention has been described with reference to itspreferred embodiments, those skillful in the art will recognize changesthat may be made in form and structure which do not depart from thespirit of the invention.

Alternatively, combinations of cage materials could be beneficial(i.e.,—a ceramic bottom half with a PEEK top half).

We claim:
 1. An intervertebral fusion cage, comprising: a) a firstcomponent having an outside surface configured to engage an uppervertebral endplate, an interior surface opposite the outside surfacealong a first direction, and a tapered first distal end, wherein thefirst component defines a recess that extends into the interior surfacetoward the outside surface; b) a second component having first andsecond opposed surfaces that are opposite each other along the firstdirection, and a second distal end, wherein the second componentincludes a projection that extends from the first surface along thefirst direction away from the second surface, wherein the first surfaceis slidable along the interior surface in a second directionperpendicular to the first direction such that the second distal endtravels toward the tapered first distal end until the projection isreceived in the recess so as to impede the first surface from furthersliding along the interior surface in the second direction, and whereinthe projection defines first and second ends that are offset from eachother a linear distance along a third direction that is greater thaneach of a height of the projection along the first direction and a widthof the projection along the second direction, and the third direction isperpendicular to each of the first and second directions.
 2. Theintervertebral fusion cage of claim 1, configured to be inserted into anintervertebral space defined by the upper vertebral endplate and asecond vertebral endplate, and the second component urges the firstcomponent to move the upper vertebral endplate away from the secondvertebral endplate as the first surface slides along the interiorsurface.
 3. The intervertebral fusion cage of claim 1, wherein thesecond component defines a proximal end opposite the second distal endalong a central longitudinal axis that is oriented along the seconddirection.
 4. The intervertebral fusion cage of claim 1, wherein thesecond component defines first and second opposed side walls that arespaced from each other along the third direction, and extend from thefirst surface to the second surface, and the projection is elongate fromthe first side wall to the second side wall.
 5. The intervertebralfusion cage of claim 4, wherein the outside surface and the interiorsurface of the first component are opposite each other along the firstdirection, and the recess is elongate along the third direction.
 6. Theintervertebral fusion cage of claim 5, wherein the second direction is astraight direction.
 7. The intervertebral fusion cage of claim 5,wherein the recess extends into the interior surface along the firstdirection and terminates without extending through to the outsidesurface.
 8. The intervertebral fusion cage of claim 1, wherein the firstcomponent has a first throughhole extending from the outside surface tothe interior surface.
 9. The intervertebral fusion cage of claim 8,wherein the second component has a second throughhole extending from thefirst opposed surface to the second opposed surface.
 10. Theintervertebral fusion cage of claim 1, wherein the first component hasfirst and second opposed surfaces that extend from the outside surfaceto the interior surface, and the first component defines a plurality ofthroughholes extending from the first side surface to the second sidesurface.
 11. The intervertebral fusion cage of claim 1, wherein theoutside surface is tapered toward the interior surface at the taperedfirst distal end.
 12. The intervertebral fusion cage of claim 1, whereinthe first and second opposed surfaces are opposite each other along thefirst direction.
 13. The intervertebral fusion cage of claim 1, whereinthe first component is elongate along the second direction, and theoutside surface of the first component defines a plurality of ridgesthat are oriented generally along the third direction.
 14. Theintervertebral fusion cage of claim 13, wherein the ridges are spacedfrom each other along the second direction.
 15. The intervertebralfusion cage of claim 14, wherein the ridges are parallel to each other.16. The intervertebral fusion cage of claim 1, wherein the projectionextends linearly from the first end to the second end.
 17. Theintervertebral fusion cage of claim 16, wherein the projection extendslinearly along the third direction from the first end to the second end.18. The intervertebral fusion cage of claim 1, wherein the first surfacefaces the interior surface as the first surface slides along theinterior surface in the second direction.
 19. An intervertebral fusioncage, comprising: a) a first component having a first outside surfaceand a first interior surface that faces opposite the first outsidesurface, a first outer surface that extends from the first outsidesurface to the first interior surface, and a second outer surfaceopposite the first outer surface along a first direction, wherein thesecond outer surface extends from the first outside surface to the firstinterior surface, and the first interior surface extends from the firstouter surface to the second outer surface; and b) a housing componenthaving a second outside surface configured to engage a vertebralendplate and a second interior surface that faces opposite the secondoutside surface, a first sidewall extending from the second interiorsurface and away from the second outside surface, and a second sidewallextending from the second interior surface and away from the secondoutside surface, wherein the first side wall is opposite the second sidewall along the first direction, wherein the first component isconfigured for insertion into the housing component such that anentirety of the first interior surface is disposed between the first andsecond sidewalls with respect to the first direction, and the firstinterior surface abuts the second interior surface.
 20. Theintervertebral fusion cage of claim 19, wherein the first outsidesurface and the second outside surface are spaced from each other alonga second direction that is perpendicular to the first direction, and thefirst interior surface abuts the second interior surface when the firstcomponent is inserted into the housing component along a third directionthat is perpendicular to each of the first and second directions. 21.The intervertebral fusion cage of claim 19, wherein the housingcomponent defines a throughhole that extends from the second outsidesurface to the second interior surface, and a plurality of throughholesthat extend through the second sidewall.
 22. The intervertebral fusioncage of claim 19, wherein the first component defines an enclosedaperture is entirely disposed within a perimeter defined by the firstoutside surface, the first interior surface, the first outer surface,and the second outer surface.
 23. The intervertebral fusion cage ofclaim 19, wherein the first interior surface is parallel to the secondinterior surface.
 24. The intervertebral fusion cage of claim 19,wherein a line oriented along the first direction intersects each of thefirst and second sidewalls and each of the first and second outersurfaces.
 25. The intervertebral fusion cage of claim 19, wherein thefirst interior surface rides along the second interior surface as thefirst component is inserted into the housing.
 26. The intervertebralfusion cage of claim 19, wherein the first component is configured forinsertion into the housing component such that an entirety of the firstinterior surface is aligned with each of the first and second sidewallsalong the first direction.
 27. The intervertebral fusion cage of claim19, wherein each of the first outside surface and the first interiorsurface is planar.
 28. The intervertebral fusion cage of claim 27,wherein each of the second outside surface and the second interiorsurface is planar.
 29. The intervertebral fusion cage of claim 28,wherein the first outside surface and the first interior surface areparallel to each other and to each of the second outside surface and thesecond interior surface.
 30. The intervertebral fusion cage of claim 19,wherein the first outer surface extends continuously from the firstoutside surface to the first interior surface.